Large scale energy storage is of considerable interest for power generation and distribution systems, to assist with exploitation of capricious energy sources such as wind and solar. At the moment, the main technology in wide use for reversibly storing electric power is hydropumping—drawing electricity off the grid to pump water uphill and then letting the water back down through power turbines later on. Hydropumping is highly efficient (about 80%) but suffers from (1) the need to allocate land to build dams and lakes, (2) the need for mountains, which aren't always available nearby, and (3) the need for water.
Recent developments in solar energy have revealed the substantial cost effectiveness of storing heat in tanks of molten salt for later use in generating electricity, by means of steam turbines, when the sun isn't shining. However, these storage facilities are adapted to store solar thermal energy, and are therefore not directly applicable to the storage of wind energy, which is mechanical energy as opposed to thermal energy. Molten salt has also been used as a primary coolant in nuclear reactors. Another approach for energy storage is considered in US 2010/0251711, where hot and cold storage tanks are employed in connection with heat storage.
However, efficiency is critical for energy storage, and it is especially critical for large scale energy storage. Therefore, it would be an advance in the art to provide energy storage having improved efficiency, especially for capricious sources of mechanical energy (e.g. wind energy).